Provide more descriptive error messages when the window isn't displayable

macos/src/main/java/com/github/weisj/darklaf/platform/macos/MacOSDecorationsProvider.java

@@ -67,7 +67,11 @@ public class MacOSDecorationsProvider implements DecorationsProvider {
     public TitlePaneLayoutInfo titlePaneLayoutInfo(final CustomTitlePane customTitlePane) {
         if (!(customTitlePane instanceof MacOSTitlePane)) throw new IllegalStateException();
         long hwnd = ((MacOSTitlePane) customTitlePane).windowHandle();
-        if (hwnd == 0) return new TitlePaneLayoutInfo(new Rectangle(0, 0, -1, -1));
+        if (hwnd == 0) {
+            throw new IllegalStateException(
+                    "Window isn't displayable (but has to in order to compute the title pane layout."
+                            + " If you need this information before the window is displayed call `frame.addNotify()` to make it displayable.");
+        }
         float[] b = JNIDecorationsMacOS.windowButtonRect(((MacOSTitlePane) customTitlePane).windowHandle());
         return new TitlePaneLayoutInfo(new Rectangle((int) b[0], (int) b[1], (int) b[2], (int) b[3]));
     }

macos/src/main/java/com/github/weisj/darklaf/platform/macos/ui/MacOSTitlePane.java

@@ -200,7 +200,7 @@ public class MacOSTitlePane extends CustomTitlePane {
     }
 
     public long windowHandle() {
-        return decorationInformation.windowHandle;
+        return decorationInformation != null ? decorationInformation.windowHandle : 0;
     }
 
     protected class RootPanePropertyChangeListener implements PropertyChangeListener {

platform-decorations/src/main/java/com/github/weisj/darklaf/platform/decorations/NativeDecorationsManager.java

@@ -119,7 +119,8 @@ public class NativeDecorationsManager {
 
     public TitlePaneLayoutInfo titlePaneLayoutInfo(final JRootPane rootPane) {
         RootPaneUI ui = rootPane.getUI();
-        if (!(ui instanceof AbstractNativeDecorationsRootPaneUI)) throw new IllegalStateException();
+        if (!(ui instanceof AbstractNativeDecorationsRootPaneUI))
+            throw new IllegalStateException("UI is not of type " + AbstractNativeDecorationsRootPaneUI.class);
         return decorationsProvider.titlePaneLayoutInfo(((AbstractNativeDecorationsRootPaneUI) ui).titlePane());
     }